[POWERPC] 44x: add missing define TARGET_4xx and TARGET_440GX to cuboot-taishan

In order to get the proper boad info (bd_info) structure defined in ppcboot.h
both TARGET_4xx and TARGET_44x should be defined for all PowerPC 440 boards.
The 440GX boards also need TARGET_440GX defined since they have 4 EMACs and
there are 4 MAC addesses in bd_info passed by u-boot.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>